1. Field of the Invention
The disclosed technology relates to a method for plasma texturing of a surface of a substrate (e.g., a silicon substrate), which may be used in an industrial process for texturing a surface of silicon photovoltaic cells.
2. Description of the Related Technology
Surface texturing of photovoltaic cells is used to increase the cell short-circuit current and thus the cell efficiency by reducing the reflection of incident light. Several texturing techniques have been proposed, such as wet chemical etching, dry plasma etching and mechanical texturing.
For single crystalline or monocrystalline silicon substrates surface texturing can be obtained by using a wet anisotropic etchant, such as for example a solution of NaOH or KOH with the addition of isopropanol for wettability improvement. Typically pyramid structures with a height in the range of 3 to 5 micrometer are formed. Alkaline texturing is not effective on multi-crystalline silicon and ribbon silicon substrates because of the random crystallographic orientation of the silicon lattice and because of the crystal orientation dependence of the etching process.
Acidic texturing or iso-texturing of multicrystalline silicon substrates can for example be obtained with acid mixtures based on HF and HNO3 in CH3CO2H or in water. The acid texturing requires defects at the surface to initialize the etching process. Thus, for example saw damage can be removed simultaneously with creating a surface texture. Acidic texturing has some disadvantages: the texture formation relies on surface damage, it is difficult to create single side surface texturing, acid texturing removes several micrometers of silicon material (typically 5 to 7 micrometer at each side) and is therefore difficult to apply on thin film silicon solar cells such as epitaxial cells wherein the thickness of the epitaxial layers is typically in the range between 2 and 6 micrometer, and chemical waste treatment is required for the hazardous etch products.
In some photovoltaic technologies, e.g. technologies with rear-side surface passivation such as the i-PERC technology, there is a need for single side texturing, wherein only the front side of the photovoltaic cells is textured. Wet texturing processes such as alkaline texturing and acidic texturing are not well suited for single side texturing.
Mechanical texturing, based on forming grooves with a diamond saw blade, has been proposed for single side texturing. However this method leads to high mechanical stresses and metal interruptions due to the deep structures. Furthermore, it is not cost effective, has a low throughput and is not applicable for very thin solar cells.
Plasma texturing offers a method that allows single side surface texturing, that allows low silicon removal rates and thus may be used on thin cells, that can be used on damage-free substrates, that is independent on crystal orientation and that has high throughput and low cost of ownership. RIE (Reactive Ion Etching) plasma texturing methods, based on ion bombardment of the silicon surface, have been developed using SF6 and O2 (e.g. U.S. Pat. No. 6,091,021 and U.S. Pat. No. 6,329,296) or using SF6, O2and Cl2 (e.g. U.S. Pat. No. 7,128,975). However, it is known that RIE plasma texturing causes damage to the silicon surface due to ion bombardment, resulting in a significant degradation of the Internal Quantum Efficiency and the open-circuit voltage. This is for example reported by S. H. Zaidi et al in “Characterization of random reactive ion etched-textured silicon solar cells”, IEEE Transactions on Electron Devices, Vol. 48, No. 6, 2001. Therefore, after RIE plasma texturing, there is a need for removing the surface damage caused by the texturing step. The most effective means of recovering a damage-free silicon surface was found to be isotropic wet chemical etching, thereby removing the damaged layer. However, in an industrial environment the need for such a wet etching may be an important drawback.
In “Plasma texturing processes for the next generations of crystalline Si solar cells”, Proceedings of the 21st European Photovoltaic Solar Energy Conference, 2006, p 754, H. F. W. Dekkers et al. report a plasma texturing process based on microwave plasma excitation with low ion flux and low impact energy on the substrate. In this way ion impact on the substrate can be substantially reduced as compared to e.g. RIE plasma texturing, and surface damage caused by ion bombardment can be substantially avoided. However, it is shown that after microwave plasma texturing using an SF6/N2O plasma a sponge-like texture with varying quality over the substrate area is obtained. It is suggested that an industrial process with large throughput and with a good uniformity over large areas can be obtained by adding a second plasma etching using a Cl2 plasma. Using this second step may transform the surface roughness induced in the first step into a more uniform texture, based on an anisotropic etching effect. However, it is a disadvantage of this method that Cl2 is used, a hazardous, toxic and corrosive gas.